This post is inspired by the problem that fans of science often have with the reasonableness of holding Chrisitan religious beliefs, and the twin problem that makes Chrisitans often nervous about applauding the sciences. My case is that both problems often result in failing to distinguish the properly separate spheres of scientific knowledge and religious knowledge.
Science, everyone agrees, arrives at truth-claims by using the apparatus of the natural world, in order to make conclusions about the things in the natural world. That is, science as a human endeavor is concerned about testable physical causes and reactions in the world of matter, both living and non-living. Even when its hypotheses are not actually testable, science thinks that it's conclusions are at least testable in principle, given enough time, money, improved technology, etc. Christians, of all people, should be glad and optimistic about the possibility and value of scientific conclusions because they believe (on scriptural grounds, no less) that God has made and approves of the physical world, and that he has made it in in an orderly way that generally conforms to basic physical laws. To put this another way, Christians have deep theological reasons (deeper than some other religious traditions) to be champions of the knowledge that science provides.
But science is not in the business of commenting on the role of non-physical realities, like God. It simply has no tools to weigh in on questions like "Does God exist?" "What is the meaning of life?" and, "What's the difference between right and wrong?" If such questions are meaningful at all, they are not meaningful because of any objects or events in the physical world. This in no sense makes science an enemy of such religious questions, but serves to remind us that such questions cannot be settled with the physical tools of the laboratory. Problems result when fans of science (against the advice of most scientists) make this kind of a move: "Unless science can show me evidence for why a certain religious belief is true, it is unreasonable for me to believe it." Philosophers would call this a "category mistake." Religious knowledge, whatever it is, is not derived by the rules that properly govern the formation of scientific knowlege.
In everyday life we understand that there are different types of knowledge, some of which allow us to reasonably hold beliefs that we have no scientific ground for. My favorite example of this is the inutitive knowledge of the "hunch". When we say that we have a hunch about something, we specifically mean that we can't cite any particular evidence (and certainly not scientific evidence) for some belief that we find ourselves in possession of. It is often reasonable to act on hunches even though it is not "scientifically reasonable" to do so. While "hunch knowledge" is not as objectively testable or even as regularly reliable as "scientific knowledge", it is the fool that completely ignores her hunches for those reasons!
Is "religious knowlege" like "hunch knowlege"? In many ways "no", but in one important way "yes": both show an alternate and reasonable means of gaining certain beliefs that are not the result of scientific inquiry. If an invisible and non-material God exists, science (by definition) will have no evidence to offer one way or another. If God does exist, and I come to believe that he does, the reasonableness of my belief will be grounded on different criteria than the canons of the scientific method. Again, saying this does not slight science in any way.
Oftentimes the secular critique of religious belief is on the grounds that we are irrationally dismissing the gains of scientific knowledge if we come to hold any untestable religious beliefs. But this is to wrongly think that all knowledge must be scientifically derived. Science itself would place no such limits on us and how we form proper beliefs. The opposite mistake is the sometimes Christian hostility toward the sciences that results from forgetting that God made a physical world that has observable, testable processes that can be discovered without reference to the supernatural. It is not unspiritual to pursue the sciences -- if anything, it honors the God who set the world up in such a way that invites us to examine its mechanisms and complexity. While some religious beliefs refer to physical things and historical events (especially in Christianity), including the miraculous suspension of normal physical laws, the basic beliefs of Christanity are not unreasonable by any judgment from science.
The last sentence will raise in some minds a good question about the rules for another category of belief, "historical knowledge". On what grounds to can we claim that a certain event happened in a certain way and for certain reasons? History deals with events in the physical world that are, therefore, somewhat open to scienfic conjecture -- but not fully, given the amazing number of unknown variables and unobservabe causes that go into any one event. Events of ancient history are even harder to study, given the often paucity of sources. It's not hard to see why history, as an academic discipline, has sometimes been considered a science, and at other times an art. Since Chrisitanity rises or falls based on whether or not certain events took place in first-century Palestine, several important Christian beliefs should be considered claims to "historical knowlege". Here, things get more interesting, because both Christians and non-believers are properly operating on the same turf, dealing with the same category of knowlege. The reasonableness of the Christian beliefs about the life, death, and resurrection Jesus Christ is based, in part, on whether or not Christians are using the canons of historical knowledge propery or improperly, and, whether they have the right canons at all. This is a much more slippery debate, given the slippery nature of historiography in general. Yet (I think) the Christians make a strong case, though the reasons for saying so will require another post.